Issue 96: The Basis for Recommending the 520 Hz Fire Alarm Signal

Smoke and other fire alarms are intended to warn people of the
presence of an unwanted fire so that they can take appropriate action
such as extinguishing the fire (if practical, and appropriate means are
available) or evacuating (themselves and others they are responsible for
such as children or sick or elderly adults) from the building to avoid
being injured or killed by the fire. To do this the detector component
of such alarms must reliably detect a fire as early as possible while at
the same time avoiding being triggered by phenomena other than a fire.
To be effective, upon fire detection, alarms must provide a warning
signal but remain "silent" otherwise. Usually the signal is a loud and
recognisable sound which must be noticed quickly by people so that they
can react as soon as possible. Fire alarm warnings may be beneficial at
any time but it is obvious that people are most vulnerable, and the
warning most valuable, when they are asleep and therefore alarm signals
must be particularly effective when people are asleep. In this article
we are concerned only with alarm sounds. Other warning signals are used
for people who are hearing impaired.

It has been shown by us and others (for example Nober et.al. Fire Journal July 1981 and Kahn Fire Technology 1984)
that relatively low level sounds of various types will awaken many
sleeping people, but it is obvious that to awaken most people alarm
sounds should be very loud in locations where people sleep (mainly
bedrooms and lounge rooms) and, at a given sound level or volume, should
be noticed and acted on by as many people as possible. Previous
research, confirmed by our research, has shown that different sounds at a
given measured sound level (dBA) will awaken sleeping people in
different proportions, depending on a variety of characteristics as well
as individual differences.

The groups of sleeping people tested have included children,
unimpaired adults, adults impaired with alcohol, adults impaired with
hypnotics (sleeping tablets), adults aged over 65 years and adults who
are hard of hearing. Other than the unimpaired adults, these are people
who are unusually vulnerable to injury and death from fire (particularly
while they are asleep).

Many different sounds have been used in our testing of the response
of these people while sleeping. These sounds include the current smoke
alarm signal used in Australia, the US and many other countries (an
~3100 Hz pure tone), pure tones of 400, 800 and 1600 Hz, 400, 520, 800
and 1600 Hz square wave sounds, white noise, whooping sounds (one
varying between 400-1600 Hz and another between 400-800 Hz) and voice
messages (child's mother's voice, voices of male and female actors). In
all of these tests the 520 Hz square wave has been found to be as good
or better at waking people than the others while the current smoke alarm
sound has consistently been found to be the worst of those tested.

A useful comparison of the current smoke alarm sound (~3100 Hz tone)
with the 520 Hz square wave sound when presented at the same level at
the pillow (75 dBA except as noted below) compares the proportion of
people in the various groups who slept through the alarm. (The
proportion that slept through represents the people who, because they
are unresponsive, remain vulnerable to the fire even though the alarm
has sounded.) The comparison uses the ratio of the proportion that did
not respond to the current smoke alarm sound and the proportion that did
not respond to the 520 Hz square wave sound. Thus a ratio of 1:1 means
equal proportions, and a ratio of 12:1 means that 12 times as many
people did not respond to the current smoke alarm sound compared with
the 520 Hz square wave. In the following, N = number of people tested
and BAC = Blood Alcohol Concentration. Thus of:

adults aged >65 years (N = 10) under the influence of their
prescribed hypnotic 17% slept through the current alarm, 0% slept
through the 520 Hz square wave (ratio = infinity)

This comparison makes it obvious that at the same sound level (75
dBA) the 520 Hz square wave is far superior to the current alarm sound
for waking all of these vulnerable people.

An explanation for this superiority involves the complexity of a
square wave sound. The 520 Hz square wave has been described as being a
dissonant noise. It has a fundamental frequency of 520 Hz and subsequent
peaks at the 3rd, 5th, 7th etc. harmonics. These multiple peaks are
less likely to be masked by ambient noise than a single frequency.
Several of the peaks are in the frequency range where human hearing is
most sensitive, are also more than a critical bandwidth apart, and the
different frequencies activate different parts of the basilar membrane
in the inner ear thus increasing the perceived loudness.

More detailed information on the research and the results is available in the following references (* = review papers)

*Thomas, I. and Bruck, D. (2009). Awakening of Sleeping People – a Decade of Research Fire Technology, 46 (3), 743-761.

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